This application is the National Stage of International Application No. PCT/DE96/02183, filed Nov. 14, 1996. Benefit of priority to 35 U.S.C. xc2xa7365(b) to German application no. 195 42 795.5, filed Nov. 16, 1995 is claimed herein.
The invention relates to a method for the quantification of tumor cells in a body fluid, in which firstly a reaction is carried out with the sample to be investigated, in which reaction the RNA component of telomerase is specifically amplified, and subsequently the amount of amplified nucleic acid is determined quantitatively, and to test kits suitable therefor.
Virtually all solid malignant tumors have the potential to form metastases. The metastasis process comprises the spread of malignant cells as micrometastases, usually via the blood or lymph to remote organs and the development of autonomous secondary tumors. The extent of metastasis determines the prognosis of an oncosis.
The requirements of tumor prevention or aftercare programs are to diagnose primary tumors or a recurrence or a metastasis early, even before metastases become clinically manifest. This aim cannot yet be satisfactorily met with the available instrumental techniques; in particular, there is still a diagnostic gray zone between circulating tumor cells and incipient formation of metastases in organs. Early diagnosis of circulating malignant cells, for example in peripheral blood of a patient undergoing tumor aftercare would make it possible to apply immunomodulating therapy or polychemotherapy, which is possibly curative, at an early date, that is to say even before organ metastasis becomes manifest. Quantification of the metastases in peripheral blood before and after the therapy represents an important control in such cases.
GB 2 260 811 proposes, for example, a diagnostic method for detecting malignant tumors which are associated with normal cells of a particular body tissue, where the normal cells form at least one gene product specific for this tissue. In this detection method, body fluid, for example blood, in which the cells do not normally occur in a healthy person, is taken from the patient, and the mRNA of the specific gene product is amplified and detected. An example mentioned is tyrosinase for detecting melanoma cells in peripheral blood. However, the disadvantage of this method is that it is linked to tissue-specific gene products, does not allow quantification of the melanoma cells and, moreover, gives false-positive results.
Kim et al. describes the results of an assay with which it was possible to determine telomerase activities in tumor tissues [Kim et al. (1994). Science 266: 2011]. The telomerase activity was detected with a sensitivity of about 1 immortal cell/104 normal cells in 98 of 100 cancer cell cultures and 90 of 101 malignant tumors, and in germinal tissues, but not in 22 normal somatic cell-cultures.
Telomerase is a newly described ribonucleo-protein with reverse transcriptase activity [Shippen-Lentz et al. (1990), Science 247: 546] which uses an integral RNA sequence as template for independent DNA synthesis [Greider et al. (1989). Nature 337: 331] by which new telomeric DNA are synthesized at the ends of the chromosomes. Telomeres consist of highly conserved (TTAGGG)n in tandem sequences with a length of about 5-15 kilobases (kb)/cell genome and have the task of stabilizing the chromosomes on the nuclear membrane and protect the coding genomic DNA from uncontrolled recombination and degradation [Mehle et al. (1994). Cancer Res 54: 236]. Whereas a dynamic equilibrium between shortening of the chromosome ends and de novo synthesis of telomeric sequences by telomerase is postulated in lower eukaryotes, normal human somatic cells show low or undetectable telomerase activity. In addition, telomerase is not growth-regulated, in contrast to other DNA enzymes, since none of the actively proliferating cell cultures showed detectable telomerase activity. Only germ cells and almost all tumor cell lines [Ohyashiki et al. (1994). Cancer Genet Cytogenet 78:64; Rogalla et al. (1994). Cancer Genet Cytogenet 77: 19; Schwartz et al. (1995). Cancer 75: 1094] and tumor tissues (Lunge, [Hiyama et al. (1995). Oncogene 10: 937; Shirotani et al. (1994). Lung Cancer 11: 29], kidneys [Mehle et al. (1994). Cancer Res 54: 236], ovaries [Chadeneau et al. (1995). Cancer Res 55: 2533] and blood [Counter et al. (1995). Blood 85: 2315]) show measurable telomerase activity and a constant telomere length which is retained throughout an infinite number of cell divisions. Activation of telomerase with the stabilization, associated therewith, of the telomere length can therefore be regarded as a critical step in the direction of immortalization of somatic cells.
Feng et al. were able to clone the integral RNA sequence of human telomerase (hTR), which is encoded on the distal segment (q) of chromosome 3. The authors were able to demonstrate, by competitive polymerase chain reaction (PCR), a significant increase in telomerase expression in tumor tissues and in germinal tissues by comparison with normal somatic cells [Feng et al. (1995), Science 269: 1236]. An antisense construct of the hTR sequence caused cell death (apoptosis) in transfected HeLa cells. These data demonstrate stringent repression of telomerase in somatic tissues, as well as the fact that malignant growth depends on the presence of immortal cells and on the activation of telomerase.
The object of the present invention was therefore to develop a method with which it is possible to determine tumor cells quantitatively in a body fluid.
The invention therefore relates to a method for the quantification of tumor cells in a body fluid, in which firstly a reaction is carried out with the sample to be investigated, in which reaction the RNA component of telomerase is specifically amplified, and subsequently the amount of amplified nucleic acid is determined quantitatively, and to test kits suitable therefor. Body fluid means for the purpose of the present invention, for example, blood, urine or else stool, exudates or transudates from body cavities, especially peripheral blood.
Peripheral blood is, for example, taken from the subject by puncturing an artery, vein or finger pad and is transferred into an RNA lysis buffer which comprises, for example, urea or, preferably, guanidinium isothiocyanate, in order to denature any RNases present and to release the nucleic acids from the cells [see, for example, Chomczynski et al. (1987) Anal. Biochem. 162, 156]. The nucleic acids can be isolated from the strongly saline medium of the RNA lysis buffer, for example, by means of silica particles to which all nucleic acids are able to bind [Boom et al. (1990) J. Clin. Microbiol., 29, 495]. The particles are then washed several times with suitable buffer and the bound nucleic acids are eluted. It is subsequently advantageous to hydrolyze any genomic DNA present in the sample using RNase-free DNase in a suitable buffer, so that no false-positive results or excessive background noise result due to false amplification signals, because DNA is possibly still present, in the later amplification of the RNA components of telomerase. This is generally followed by inactivation of the DNase, for example by phenol extraction and/or heat denaturation. It is possible and advantageous, before or, preferably, after treatment of the sample with DNase, also to purify the RNA present in the sample further, for example by chromatographic methods such as ion exchange chromatography, preferably on silica gel.
To check whether possibly interfering genomic DNA is still present in the sample, it is subsequently possible to carry out an amplification reaction with the telomerase-specific oligonucleotide primers which are described hereinafter, in which case the RNA present in the sample is not transcribed to cDNA by a reverse transcription reaction beforehand. Only in the case where the sample is free of telomerase-specific DNA does no amplification take place, with the result that no amplified DNA can be measured.
This is followed by transcription of the RNA present in the sample into cDNA, generally by means of the reverse transcription reaction, for example with AMV reverse transcriptase. The method is generally known and is described, for example, in Sambrook et al., Molecular Cloning: A Laboratory Manual, New York Cold Spring Harbor Laboratory, 1989. In a preferred embodiment of the reverse transcription, it is also possible to use a thermostable RNA-dependent DNA polymerase as described in WO 90/07641. Suitable oligonucleotide primers for the reverse transcriptase are, for example and advantageously, the oligonucleotide primers described below or random primers with a particular length.
The subsequent amplification can be carried out, for example, with a DNA polymerase, for example by the polymerase chain reaction (PCR) (see, for example, U.S. Pat. Nos. 4,683,195; 4,683,202; 4,965,188) or, preferably, with an RNA polymerase by, for example, isothermal nucleic acid sequence-based amplification (NASBA). Specific oligonucleotide primers derived from the nucleic acid to be amplified are required for the amplification in each case. It is possible in the present invention to use any sequence section of the RNA component of telomerase for synthesizing the oligonucleotide primers. The oligonucleotide primers are preferably about 20 to about 30, preferably about 20 to about 25, nucleotides long. The amplification product is generally about 100 to about 2000 bases, preferably about 200 to about 1500 bases, in particular about 300 to about 350 bases, long. The following oligonucleotide primers, which have been derived from the sequence shown in FIG. 1, are particularly preferred for the novel method:
5xe2x80x2 GACTCGGCTC ACACATGCAG TTCGC 3xe2x80x2 (TM1) (SEQ ID NO:1), and/or
5xe2x80x2 CTGGTCGAGA TCTACCTTGG GAGAAGC 3xe2x80x2 (TM2) (SEQ ID NO:2),
where TM1 and/or TM2 may, where appropriate, additionally comprise a promoter sequence for an RNA polymerase. The oligonucleotide primer TM1 corresponds to the 5xe2x80x2 primer and TM2 corresponds to the 3xe2x80x2 primer. The amplification product is 327 bp long. The primers may, for example, be prepared synthetically using the triester methods [Matteucci et al., (1981), J. Am. Chem. Soc., 103, 3185-3191]. The DNA polymerase which can be used is, for example, a non-thermostable DNA polymerase such as T4 DNA polymerase, T7 DNA polymerase, E. coli polymerase I or the Klenow fragment of E. coli or, preferably, a thermostable DNA polymerase such as Taq polymerase (see, for example, U.S. Pat. No. 4,889,818).
The general principle of the PCR consists of heat-denaturation of the DNA and restoration of the double strand in the presence of suitable oligonucleotide primers with opposite orientation of the single strand using DNA polymerase in several repeated reaction cycles. The cycle is then repeated until sufficient DNA has been formed for quantification by one of the methods described below. In general, about 20 to about 40 cycles, preferably about 30 to about 35 cycles, suffice.
In the NASBA method (also called 3SR system) there is use of at least one oligonucleotide primer, preferably TM2, which comprises a promoter for the RNA polymerase, preferably for T7 RNA polymerase [see, for example, Guatelli et al. (1990) Proc. Natl. Acad. Sci. USA, 87, 1874-1878 or Kievits et al. (1991), J. Virol. Methods, 35, 273-286]. Firstly, as already described in detail above, the RNA is transcribed with the aid of one of the oligonucleotide primers described above and of a reverse transcriptase, preferably AMV reverse transcriptase, into cDNA. The reaction product is an RNA:DNA double strand whose RNA component is then degraded by an RNase, preferably RNase H, to give a DNA single strand. Using one of the oligonucleotide primers described above, the DNA single strand is made up to the DNA double strand using a DNA polymerase. AMV reverse transcriptase is once again a suitable and preferred DNA polymerase because this transcriptase has not only an RNA-dependent DNA polymerase activity but also a DNA-dependent DNA polymerase activity. The reaction product is a DNA double strand which comprises the promoter for, for example, T7 RNA polymerase. The RNA polymerase then synthesizes an xe2x80x9cantisensexe2x80x9d RNA strand again, and the cycle can begin again. In general, about 20 to about 40 cycles, preferably about 30 to about 35 cycles, suffice to provide sufficient amplification product, preferably xe2x80x9cantisensexe2x80x9d RNA, for the subsequent quantification.
The amplification products can be quantified by, for example, staining them with ethidium bromide and detecting and quantifying them directly, for example, in an agarose or polyacrylamide gel. However, it is advantageous for the amplification products to be labeled already during the amplification, because this achieves higher sensitivity. Examples of suitable labels are radioactive labels, biotin labels, fluorescent or electrochemoluminescent (ECL) labels. The labels are, as a rule, carried by the nucleotides as starting materials for amplification by DNA or RNA polymerase. The radiolabeled amplification products (for example PCR or NASBA products) can be detected by measuring the radioactivity, the biotin-labeled amplification products can be detected via a dye which carries avidin or streptavidin, the fluorescent-labeled amplification products can be detected with a fluorometer and the electrochemoluminescent-labeled amplification products can be detected with an ECL detector. However, the most preferred detection method is in vitro hybridization with a previously labeled oligonucleotide which is complementary to the amplification product. The oligonucleotide generally carries the labels described above, and in connection with the NASBA amplification method the hybridization probe carries an electrochemoluminescent label, preferably a tris[2,2-bipyridine]ruthenium[II] complex label [see, for example, van Gemen et al. (1994), J. Virol. Methods, 49, 157-168].
Accurate and sensitive quantification of the amplification products can advantageously be achieved by coamplification of a defined amount of one or more known nucleic acids (standard nucleic acids) [see, for example, van Gemen et al. (1993), J. Virol. Methods, 43, 177-188]. In this case, a different but exactly known amount of the coamplifying standard nucleic acid or nucleic acids is added, for example in the form of a dilution series, to the unknown amounts of the sample to be investigated, and the amplification products of the sample and one or more coamplifying standard nucleic acids are determined quantitatively in independent mixtures. Comparison of the measured results reveals the amount of the RNA component of telomerase to be determined in the sample.
It is advantageous to amplify the coamplifying standard nucleic acid or nucleic acids with the same oligonucleotide primer as the sample to be investigated and the amplification products also have essentially the same length. It is particularly preferred for the coamplifying nucleic acids to have the same sequence as the amplification product of the sample to be determined, with the exception of about 20 nucleic acids between the oligonucleotide primer binding sites, which have an arbitrary but known sequence. It is possible thereby to quantify, independently of one another, the amplification product to be determined in the sample from the coamplifying amplification product, for example by a hybridization as described in detail, for example, in Sambrook et al. (supra), using appropriately complementary labeled oligonucleotides. It is particularly advantageous if several, preferably three, different coamplifying nucleic acids are added in different concentrations to the sample, because this makes it possible to reduce the number of individual amplification reactions which would otherwise have to be carried out [see van Gemen et al. (1994) J. Virol. Methods 49, 157-168]. It is also particularly preferred to add the coamplifying nucleic acids to the RNA lysis buffer described above because it is possible thereby to exclude the effect of possible nucleic acid losses in the subsequent workup of the sample.
Suitable and advantageous coamplifying standard nucleic acids in the present invention are RNA single stranded sequences which are prepared by in vitro transcription, for example with Sp6 or T7 RNA polymerase, from constructs which comprise the DNA or cDNA of the sample to be amplified and which are in each case provided with a randomized exchange of a sequence of, for example, about 10 to about 30, preferably about 20, nucleotides.
The constructs preferably consist of a transcription vector having a binding site for Sp6 or T7 RNA polymerase between a xe2x80x9cmultiple cloning sitexe2x80x9d in which the DNA or cDNA of the sample to be amplified has been cloned. The cloned sequence can be opened by selective hydrolysis with restriction endonucleases, preferably with two different restriction endonucleases, and a fragment of a defined length can be cut out and replaced by a fragment of equal length, for example using T4 ligase. The cloned fragment may comprise replacement of a sequence of any length, for example about 10 to about 30, preferably about 20, nucleic acids and is preferably located between the oligonucleotide primer binding sites. This procedure can be repeated in order to insert other nucleic acid sequences at the same site. If no suitable cleavage sites can be found, for example because the vector is also cut, it is necessary to create artificial cleavage sites. This can take place, for example, by recombinant PCR which is described in essence by Higuchi et al. [Higuchi, R. (1988). Nucleic Acid Res 16: 7351-7367; Higuchi, R. (1990). M. Innis A. et al. eds. San Diego, New York, Berkley, Boston, London, Sydney, Tokyo, Toronto, Academic Press, Inc. 177-183] and in the experimental part of the present invention.
Preferably used for the purpose of the invention are in vitro transcribed RNA single stranded sequences of constructs which
a) comprise the entire cDNA of the RNA component of human telomerase and
b) in which a randomized exchange of a sequence of about 20 nucleotides has been introduced.
The constructs are derived from the constructs
pGEM-hTR shown in FIGS. 5a and 5b (SEQ ID NO:17)
pGEM-hTR(Ka) shown in FIGS. 6a and 6b (SEQ ID NO:18).
It is possible by in vitro transcription of the constructs with Sp6 RNA polymerase to prepare standard RNA nucleic acids which are individually 975 base pairs long and have the sequence:
(hTRKa) shown in FIG. 7 (SEQ ID NO:19)
(hTRKb) shown in FIG. 8 (SEQ ID NO:20)
(htRKc) shown in FIG. 9 (SEQ ID NO:21).
The randomized sequence in which the standard nucleic acids differ from the wild-type RNA is in this example located in position 591-610 shown in FIG. 5a. It is 20 base pairs long.
Since the standard nucleic acids differ from one another and from the wild-type sequence in this example only by a randomized and known sequence which is 20 base pairs long, the amplification products of the standard nucleic acids and of the wild-type sequence can be detected by complementary binding of labeled oligonucleotides in four separate mixtures. Oligonucleotides which are particularly suitable for specific detection of the amplified cDNA of the RNA component of telomerase (wt) and of the standard nucleic acids (hTRKa), (hTRKb) and (hTRKc) according to the present invention are the following sequences, which have been derived from the sequences shown in FIGS. 7-9:
The corresponding reverse complementary sequences are used to detect the amplified xe2x80x9cantisensexe2x80x9d RNA.
After this, the individual amplified amounts of wild-type and standard nucleic acids are determined. The unknown initial amount of the wild-type sequence can be calculated by comparing with the different amplified amounts of the standard nucleic acids when the initial amount is known (for example hTRKa: 102, hTRKb: 104 and hTRKc: 106 molecules). It is then possible to conclude from this the number of metastases for the removed body fluid.
As internal positive control of the method and of the sample to be investigated it is possible additionally to amplify and detect a nucleic acid which generally always occurs in a body fluid. Examples of suitable nucleic acids are the mRNA coding for xcex2-globin or for glyceraldehyde-phosphate dehydrogenase (GAPDH) (see, for example, GB 2 260 811) which always occur in the cells of the body fluid. Suitable oligonucleotide primers for human xcex2-globin mRNA are, for example, primers with the sequences:
where the oligonucleotide primers may, where appropriate, additionally comprise a promoter sequence for an RNA polymerase.
To prevent or reduce false-positive results or so-called background noise which is caused by telomerase activities which are possibly present in nontumor cells, it is advantageous to purify the body fluid which has been taken before the novel investigation. The intention is, in particular, to deplete stem cells and/or activated immune cells, or concentrate tumor cells, in the sample to be investigated. Since, as a rule, the individual cells have specific surface markers, removal or concentration of the cells by immunoabsorption is particularly advantageous. Examples of suitable methods are magnetic (MACS) or fluorescence-activated (FACS) cell sortings [see, for example, Gxc3x6ttlinger and Radbruch (1993) mta, 8, 530-536, No. 5]. Thus, for example, hemopoietic stem cells can be removed from the blood sample by means of MACS via their CD34 surface marker [Kato and Radbruch (1993) Cytometry, 14, 384-392]. B cells can be removed, for example, via their CD10, CD19 and/or CD20 surface markers, and T cells via CD45RA and/or CD7. Tumor cells can be concentrated via their specific tumor markers, for example CEA. Besides MACS or FACS, also particularly suitable for depletion or concentration of the relevant cells are antibodies against the specific surface markers, which are bound in particular to commercially obtainable magnetic beads (for example Dynabeads M450, Dynal Corp.).
It is also particularly advantageous, alone or in conjunction with the purification methods described above, to compare the amount of RNA component of telomerase from venous blood with the amount of RNA component of telomerase from arterial blood, since it is possible to detect, for the purpose of tumor cell determination, only about 20% of all cells in venous blood samples, compared with 100% of the cells in arterial blood samples [Koop, S. et al. (1995) Cancer Res. 55, 2520-2523]. It is likewise suitable to compare blood from the finger pad with venous or arterial blood.
Quantitative determination of the RNA component of telomerase in the sample makes it possible to determine whether tumor cells, especially metastases, in particular micrometastases, of malignant tumors are present in the body fluid, and in what quantity. This is of great use in particular for early clinical diagnosis of the formation of metastases from malignant tumors and for monitoring tumor therapy. Tumor cells which can be detected with the present invention are, in particular, tumor cells from metastases, preferably micrometastases, from malignant tumors, especially cells from metastasizing tumors and/or neoplasms which are derived, for example, from a T-cell lymphoblastoma, T-cell leukemia cells, chronic myeloid leukemia cells, acute lymphatic leukemia cells, chronic lymphatic leukemia cells, teratocarcinoma, melanoma, carcinoma of the lung, large bowel cancer, breast cancer, hepatocellular carcinoma, kidney tumor, adrenal tumor, prostate carcinoma, neuroblastoma, brain tumor, small-cell carcinoma of the lung, rhabdomyosarcoma, leiomyosarcoma and/or lymphoma.
The present invention further relates to the oligonucleotide primers with the sequence
where TM1 and/or TM2 may, where appropriate, additionally comprise a promoter sequence for an RNA polymerase;
and
an oligonucleotide with the sequence
and
a nucleic acid construct pGEM-hTR as shown in FIGS. 5a and 5b or a nucleic acid construct pGEM-hTR(Ka) as shown in FIGS. 6a and 6b;
and the standard RNA nucleic acid for coamplification of the sequence:
(hTRKa) as shown in FIG. 7 (SEQ ID NO:19)
(hTRKb) as shown in FIG. 8 (SEQ ID NO:20)
(hTRKc) as shown in FIG. 9 (SEQ ID NO:21), and the cDNAs thereof, and the oligonucleotides for detecting the amplified cDNA of the wild-type nucleic acid and of the cDNA of the hTRKa, hTRKb and hTRKc standard nucleic acids with the sequence:
and the corresponding reverse complementary sequences of the oligonucleotides for detecting the amplified xe2x80x9cantisensexe2x80x9d RNA.
The invention additionally relates to a kit for quantifying tumor cells in a body fluid, for example blood, urine or else stool, exudates or transudates from body cavities, especially peripheral blood, comprising
(a) nucleic acid or nucleic acids for the coamplification, and
(b) oligonucleotide primer pair for specific amplification of telomerase-encoding nucleic acid and of the nucleic acid or nucleic acids specified in (a), where the standard RNA nucleic acid for the coamplification mentioned in (A) has or have the following sequence:
(hTRKa) as shown in FIG. 7 (SEQ ID NO:19)
(hTRKb) as shown in FIG. 8 (SEQ ID NO:20)
(hTRKc) as shown in FIG. 9 (SEQ ID NO:21),
and/or the oligonucleotide primer pair preferably the following sequences:
where TM1 and/or TM2 may, where appropriate, additionally comprise a promoter sequence for an RNA polymerase.
The novel kit may also comprise in addition, as described in detail above, a labeled oligonucleotide as hybridization probe for specific detection of the amplified cDNA of the wild-type sequence and/or several labeled oligonucleotides as hybridization probe for specific detection of the amplified cDNA of the standard nucleic acid or nucleic acids. In addition, a novel kit for PCR amplification may additionally comprise the enzymes described in detail above, where appropriate labeled nucleotides and/or suitable buffers, such as, for example, a reverse transcriptase, preferably an AMV reverse transcriptase, a DNA polymerase, preferably a Taq polymerase and/or a DNase and, where appropriate, means suitable for depletion of stem cells and/or activated immune cells and/or for concentration of tumor cells, as described in detail above.
Another novel kit for NASBA may likewise comprise, besides the standard nucleic acids described in detail above, a labeled oligonucleotide as hybridization probe for specific detection of the amplified xe2x80x9cantisensexe2x80x9d RNA of the wild-type sequence and/or several labeled oligonucleotides as hybridization probe for specific detection of the amplified xe2x80x9cantisensexe2x80x9d RNA of the standard nucleic acid or nucleic acids. It may additionally likewise comprise the enzymes described in detail above, where appropriate labeled nucleotides and/or suitable buffers, such as, for example, a reverse transcriptase, preferably an AMV reverse transcriptase, an RNA polymerase, preferably a T7 RNA polymerase, an RNase H and/or a DNase, and, where appropriate, means suitable for depletion of stem cells and/or activated immune cells and/or for concentration of tumor cells, as described in detail above.
The following examples and figures are intended to describe the present invention in detail without, however, restricting it thereto.